This invention relates to a power assisted steering system comprising two hydraulic systems each of which is independently controlled by valve means to provide power assistance through servo motor means associated with a steering assembly or unit in the system. Such a steering system having primary and secondary hydraulic system or circuit parts is well known for motor vehicles and despite the additional manufacturing costs (as compared with single hydraulic circuit power steering) has several advantages, notably that if power assistance for one system part fails the other is available to ensure that the vehicle can be steered with assistance and that if there is encountered a heavy resistance to a steering maneuver, the valves may react in response to the steering input to provide power assistance from both hydraulic circuits in an attempt to overcome the resistance. The valves for controlling the power assistance which is available from the primary and secondary hydraulic circuits are usually carried within the steering assembly or gear and the cost of their inclusion is a considerable proportion of the overall cost of the assembly. By way of example, U.K. Pat. No. 1,597,057 discloses a power assisted steering system with two hydraulic circuits, the valve means for the primary circuit being mounted in a gear housing while the valve means for the secondary circuit is mounted in a piston which forms part of the servo motor means and in each case the valve means is of a relatively complicated structure which would be relatively expensive to manufacture, assemble and service.
It is an object of the present invention to provide a power assisted steering system having two hydraulic system parts and associated valves of the kind discussed above but which lends itself to relatively inexpensive manufacture and may be relatively simple to assemble and in operation.